1. Field of the Invention
The present invention relates to image forming apparatuses having a fixing unit that heats a recording material carrying a toner image to fix the toner image onto the recording material.
2. Description of the Related Art
In recent years, fixing units employing a film-heating method, which serve as fixing units installed in electrophotographic image forming apparatuses, such as copiers and laser beam printers, are being proposed.
This fixing unit includes a tubular film, a heater in contact with the inner surface of the film, and a pressing roller that forms a nip with respect to the heater via the film. The heater has a heat generating resistor that is disposed on a substrate and generates heat by receiving a supply of power, and a glass protection layer for protecting the heat generating resistor. At the nip, the recording material is heated while being conveyed, thereby fixing the toner image onto the recording material.
However, in the case where the heater is connected to a frame ground (hereinbelow, FG) of the apparatus via the film and the pressing roller, the above-described fixing unit employing a film-heating method has the following problem.
That is, if a surge voltage due to lightning is applied between a power supply line connected to the heater of the fixing unit and the FG, a high-potential surge voltage is applied to the heater, which may cause an electrical breakdown in the protection layer of the heater.
To overcome the above-described problem, Japanese Patent Laid-Open No. 6-051659 discloses a configuration in which a resistor is connected between a pressing roller and an FG. This configuration can divide and reduce the voltage applied to the protection layer of the heater when a surge voltage is applied between the power supply line and the FG, and hence, prevents an electrical breakdown.
However, in the configuration disclosed in Japanese Patent Laid-Open No. 6-051659, the clearance between the film and the FG, such as a metal frame, or the clearance between the pressing roller and the FG, such as a metal frame, must be sufficiently large to prevent a discharge from occurring when a surge voltage is applied.
That is, the above-described clearance must be set such that the impedance of a path in which a discharge occurs across the clearance between the film and the metal frame and leads to the FG is larger than the impedance of a path leading from the pressing roller to the FG via the resistor.
This is because, if a discharge occurs between the film and the metal frame, the advantage achieved by the resistor provided between the pressing roller and the FG to divide the voltage cannot be obtained, and a high-potential surge voltage may be applied to the protection layer of the heater, causing an electrical breakdown.
Thus, the configuration disclosed in Japanese Patent Laid-Open No. 6-051659 involves an increase in size of the apparatus to avoid an electrical breakdown in the protection layer.